IV 



PHYLUM CCELENTERATA 145 



fixes itself to the manubrium of one of the Trachymedusse which 

 serves as its host, and develops into a hydrula. But the latter, in- 

 stead of itself becoming metamorphosed into a medusa, retains the 

 polype form and produces other hydrula? by budding, these last 

 becoming converted into medusoe in the usual way. 



ORDER 3. HYDROCORALLINA. 



The best-known genus of Hydroid Corals is Millepora, one species 

 of which is* the beautiful Elk-horn Coral, M. alcicornis. The dried 

 colony (Fig. 103 A) consists of an irregular lobed or branched mass 



* * 



' 





FIG. 103. IMEillepora alcicornis. A, part of skeleton, natural size ; B, portion of surface, 

 magnified ; C, vertical section, magnified ; d.p. dactylopores ; g.p. gastropores ; tb. tabulae. 

 (After Nicholson and Lydekker.) 



of carbonate of lime, the whole surface beset with the numerous 

 minute pores to which the genus owes its name. The pores are 

 of two sizes : the larger are about 1 or 2 mm. apart, and are called 

 gastropores (B, g.p) ; the smaller are arranged more or less 

 irregularly round the gastropores, and are called dactylopores (d.p). 

 The whole surface of the coral between the pores has a pitted 

 appearance. Sections (C) show that the entire stony mass is 

 traversed by a complex system of branched canals, which com- 

 municate with the exterior through the pores. The wide vertical 

 canals in immediate connection with the gastropores are traversed 

 by horizontal partitions, the tabulae (tb). 



In the living animal each pore is the place of origin of a zooid : 

 from the gastropores protrude polypes (Fig. 104, P) with hypostome 



